Method and apparatus for detecting concealed substances

ABSTRACT

A method and apparatus detects the presence of a number of substances such as explosives or drugs in a container such as luggage (10) at an airport. The containers, travelling on a conveyor (15) for example are irradiated with fast neutron and gamma source radiation (6) preferably simultaneously and preferably of different frequencies. The radiation is detected (7) and the extent to which each species of the source of radiation is transmitted through the container is measured. The measurements are analyzed with reference to the known characteristic attenuation coefficients and density properties of those substances for each species of the source radiation.

TECHNICAL FIELD

This invention relates to a method and device for detecting concealedsubstances such as explosives or drugs, for example in luggage at anairport.

Explosives are occasionally concealed in luggage and parcels byterrorists for example, and smuggled through airports despite theefforts of customs officers. These devices are often not found becausethe daily volume of luggage and cargo is such that manually searchingevery item is simply impractical.

The Lockerbie disaster in 1988 highlighted the danger posed by Semtex, aplastic explosive that is difficult to detect by conventional means,particularly when formed into thin sheets. It is believed that the bombcontained only 1/2 kg of Semtex which was packed into a portable radio.

The U.S. Federal Aviation Administration (FAA) is searching for a meansof detecting concealed explosives and has set minimum requirements forsuch a detection system.

A balance must be struck between the risk of not detecting an explosiveand the delay and disruption caused by searching and false alarms sincewhile the detonation of an explosive device in an airport or aircraft isa rare event, delays and disruptions are daily concerns.

BACKGROUND ART

Most explosives are characterized by high nitrogen and oxygen contentand low carbon and hydrogen content. They are also usually of highdensity. Aware of this, the FAA began funding tests of thermal neutronactivation (TNA) in 1985. TNA involves the use of a radioactive sourcesuch as Californium-252 which emits neutrons. The neutrons are slowed ormoderated in materials high in hydrogen such as polyethylene (at whichstage the neutrons are "thermalized") and are then absorbed by theobject of interest. The absorption leads to the emission of gamma rayswhich are characteristic of the elements present.

Analysis of these rays provides information as to the nitrogen contentof the object bombarded. While explosives characteristically have a highnitrogen content, so do other materials such as certain plastics, silkand nylon which are commonly contained in luggage. Unfortunately TNAscreening devices cannot distinguish between explosive materials andthese non-explosive materials and so false alarms are often raised whichcan cause considerable delays. Also, TNA scanning devices require a veryintense neutron source and extreme measures are needed to shield airportstaff and travellers from the radiation.

The sensitivity of these devices is also less than desirable butimproving that would increase the incidence of false alarms.Furthermore, TNA scanning devices are about the size of a small car, canweigh of the order of 10 tonnes and cost about $1 million each. Also, ithas been suggested that between 300 and 700 such units would be requiredto deal with the demands of large international airports in the UnitedStates at a cost of $500 million for the machines plus housing andoperating costs of at least $92 million a year.

Dual beam X-ray machines are being field tested. They can detect organicmaterials, such as explosives, with one beam and inorganic materials,such as metals, with the other beam.

Hand-held vapour sniffers are also being tested. These take in air andidentify molecules in terms of their vapour pressure, atomic weight andliquid solubility by chromographic means.

Plastic explosives such as Semtex however have a lower vapour pressurethan TNT so they can be difficult to detect by such means.

Computerised tomography or CT scanning commonly used in medicaldiagnosis and research has been applied to the problem by scanning foran object's density, total mass and indicating its atomic number andcomposition. But as Dr. Grodzin of MIT explained at an internationalmeeting (International Conference on Accelerators in Industry andResearch, Denton, Tex., Nov. 5-9, 1990): "More than a dozennuclear-based techniques have been proposed for rapidly scanning airportluggage to find hidden explosives by measuring their elementaldistributions. In most almost every scheme, the technological challengeis the accelerator, which must produce its intense beams of neutrons andphotons . . . in an airport environment, perhaps even in an airportconcourse".

DISCLOSURE OF THE INVENTION

The object of the invention is to provide an improved or at leastalternative method and apparatus for detecting concealed substancesincluding particularly explosives.

In broad terms the invention comprises a method for detecting thepresence of a number of substances in a container, comprisingirradiating the container with fast neutron and gamma source radiation,measuring the extent to which each species of the source radiation istransmitted through the container, and analysing the measurements withreference to the known characteristic attenuation coefficients anddensity properties of the substances for each species of the sourceradiation.

The invention also comprises apparatus for detecting the presence of anumber of substances in a container, which comprises means forirradiating the container with fast neutron and gamma source radiation,means for measuring the extent to which each species of said radiationis transmitted through the container, and means for analysing themeasurements with reference to the known characteristic attenuationcoefficients and density properties of the substances for each speciesof the source radiation.

In this specification and claims "container" includes suitcases, bags,packages, boxes, parcels, freight containers and containers of any typefor carrying luggage or goods or the like, and in particular"containers" that are moved through airports, rail and shippingterminals, postal distribution centres and the like.

Also, in this specification "substance" is intended to include anysubstance that is desired to be detected if present in a container, butthe method and device of the present invention will be described interms of detecting explosives or drugs only.

Preferably the neutron and gamma radiation are emitted substantiallysimultaneously and they have different flight times to the detectors, ordifferent responses in the detectors so they can be readilydistinguished.

Preferably the source radiation is obtained from a radioactive isotopicsource such as ²⁵² Cf or Am--Be, perhaps by means of a particleaccelerator/target system.

Preferably the device of the present invention has means for conveying anumber of containers between the irradiating means and the transmissionmeasuring means.

Preferably the transmission measurements are analysed by a dataprocessor and the results are displayed on a monitor.

Preferably the transmission measurements are combined with datacollected by TNA measuring equipment and/or x-ray measuring equipment.

Preferably the device has an alarm system that is triggered when one ofthe substances is detected.

When the substance for detection is an explosive substance qualitativeanalysis or detection can be made on the basis that explosive substancescharacteristically have a high nitrogen and oxygen content but a lowcarbon and hydrogen content. The thickness or the amount of explosivesubstance is not required to be known, thus Semtex sheets are no lesseasily detectable than Semtex sticks or lumps.

Preferably, neutron and gamma radiation are simultaneously transmittedthrough the container and the reduction of intensities of the tworadiations together is analysed to yield information of materialthickness and mass attenuation coefficients. We refer to this method asthe "NEUGAT" (trademark) method (is neutron/gamma transmission).Preferably a computer is employed to display such informationgraphically for example by plotting characteristic explosive compositionagainst density so that action can be taken if an explosive-likesubstance is detected.

Each scan can be analysed as a combination of hypothetical explosive andnon-explosive material or, drugs and non-drugs material. This ispossible since on average the density of the contents of a suitcase isabout 0.2 g/cm³ whereas the densities of explosive substances and drugsare typically about 1.6 g/cm³ and about 1.0 g/cm³ respectively.

Complementary techniques can also be incorporated. For example, positronemission tomography and x-ray scanning can be used in addition to anuclear technique with neutrons.

The neutron and gamma radiation can be provided from any suitablesource, for example a radioactive isotope source such as ²⁵² CF orAm--Be or a nuclear particle accelerator target system. All sources canbe kept well within the limits required for operation in a publicwork-place. It also provides sufficient precision when detectingexplosives since the neutron absorption coefficients vary by onlyapproximately 20%.

New Zealand patent specification 213777/214666 discloses a method and anapparatus for quantitatively analysing a mixture of two or morecomponents, such as meat and fat, using neutron/gamma transmissionscanning technology. The disclosure of that specification is to beincorporated by reference into the specification of the presentapplication.

According to New Zealand patent specification 213777/214666, to obtainquantitative analysis of an n-component system, at least ndistinguishable species of radiation are required and a mathematicalanalysis is made by solving the n simultaneous equations describing theextent of transmission of each species of radiation to find the weightfraction of each component. This is described in more detail as follows:

Radiation is passed through the object being measured and thetransmitted radiation intensities are measured simultaneously by asuitable detector or array of detectors that is/are connected to anelectronic measuring and computing apparatus. In a layered system, amore complex analysis can be made by considering the known length ofmaterial through which the radiation passes and the densities of thelayered components so that the depths of each layer can be measured.

Each component of the object mixture has a different known massattenuation coefficient for each species of source radiation. Theattenuated intensities (countrates) of neutron and gamma radiation mustalso be measured when there is no object in the beam to account forbackground radiation.

When there is an object in the beam the intensities of the neutron andgamma radiation received by the detector(s) can be described in terms ofa two-component system for example as follows:

    I.sub.n =I.sub.no exp [-(u.sub.na M.sub.s +u.sub.nb M.sub.b)](A)

    Iγ=Iγ.sub.o exp [-(uγ.sub.a M.sub.a +uγ.sub.b M.sub.b)]                                                 (B)

wherein I_(n) and I are the countrates of neutron and gamma rays afterpassing through the object; I_(no) and Iγ_(o) are their unattenuatedcountrates; μ_(na), μ_(nb), μγ_(a) and μγ_(b) are the neutron and gammaray mass attenuation coefficients of components a and b, for the sourceradiation employed; and M_(a), M_(b) are the mass thicknesses (mass perunit area in the beam) of the components a and b.

Solving these two simultaneous equations (A and B) will give the valuesof M_(a) and M_(b) and the weight fraction, for example, of component a,is then simply M_(a) /(M_(a) +M_(b)).

It should be noted that it is not necessary to know the thickness of theobject to make this simple calculation. And the measurement can beintegrated over the object volume sensed by the detector or detectorarray to give the overall weight fraction for that volume.

DESCRIPTION OF THE DRAWINGS

A preferred form of apparatus of the invention is diagrammaticallyrepresented in the accompanying drawings by way of example only and theyare not intended to limit the scope of protection sought which isdefined by the claims. In the drawings:

FIG. 1 shows the preferred form apparatus for detecting the presence ofsubstances such as explosives or drugs in suitcases, travel bags andother containers of luggage that often pass through the customs sectionof an airport.

FIG. 2 is a cross-sectional view through the preferred form apparatus ofFIG. 1.

FIGS. 3 and 4 represent time of flight spectra in the monitor anddetectors of the preferred form apparatus of the invention.

FIG. 5 is a graphical representation of the pulsing of radiation bymeans of a pulsed accelerator.

FIG. 6a represents the calculated composition expressed as a weightpercent (wt %), W, against density, RHO, and shows an operational linecalled the "NEUGAT" line which in general will be a curve.

FIG. 6b is similar to FIG. 6a but shows a hypothetical separation ofexplosives, drugs and clothes signals on which basis a customs officercan decide whether a suitcase should be manually searched.

FIG. 7 represents a graphical representation similar to that shown inFIG. 6 except the nitrogen gamma count rate is additionally shown bymeans of a third axis.

The preferred form apparatus 1 shown in FIGS. 1 and 2 has a detectionunit 5 through which suitcases, travel bags and other containers 10 canbe passed by means of a conveyor 15 in an airport, rail or shippingterminal, or the like.

The detection unit 5 houses means for irradiating a container 10 withinthe detection unit 5 with simultaneous fast neutron and gamma radiation.Sources 6 of neutron and gamma radiation are surrounded by radiationshielding within the detection unit 5. A number of detectors 7 are belowthe conveyor 15 carrying containers such as luggage as shown. Thedetectors 7 are supported by detection equipment 20 which includes photomultipliers 21. The detectors and the detection equipment 20 enable eachspecies of said radiation to be detected after transmission thereofthrough the container 10 that is being analysed. The detection equipment20 is conveniently located below the detection unit 5 and the containers10 pass through the detection unit 5 one by one between the irradiatingmeans and the detectors.

Data processing facilities such as a computer 25 are connected to thedetection unit 5 so that data collected by the detectors of thedetection equipment 20 is processed by the computer 25 and displayed bya monitor 30 in a form an operator stationed at the detection unit 5 canreadily interpret. If the operator having considered the informationdisplayed by the monitor 30, suspects the suitcase or other container 10contains an explosive substance the operator simply stops the conveyor15 and takes whatever action is required.

The use of the apparatus 1 avoids the delay and disruption caused bymanually searching all or selected containers. The apparatus 1 may befully automated so that an operator is alerted when the computer 25,programmed with appropriate software, recognises an explosive-likesubstance in a suitcase or other container 10.

The detection unit 5 contains NaI(T1) [sodium iodide] or B90 [bismuthgermanate] scintillation detectors.

The detection equipment 20 may conveniently comprise, for example, anumber of Nuclear Enterprises NE213 liquid scintillators or Bicron BC501 liquid scintillators (an organic scintillator utilizing protonrecoil and pulse shape discrimination), in a cell 15 cm in diameter and15 cm in thickness, coupled to Philips XP2041 or RCA 8854photomultiplier tubes. Standard electronic measuring equipment can beused. Such a system, used in conjunction with an Am--Be source of about10 curie or a 25 μg Californium-252 source.

The time of flight spectra in the monitor 30 and detectors arerepresented in FIGS. 3 and 4 of the drawings.

The preferred form apparatus of the present invention operates byscanning the container with fast neutrons and gamma rays to measure thetransmission properties, simultaneously measuring the characteristicγ-rays from neutron capture on nitrogen and combining information oncontainer mass and volume. In this way, regions of the scan that arecharacteristic of explosive substances can be identified.

Lower neutron fluxes are used in comparison to present machines basedwholly on the ¹⁴ N(n,γ)¹⁵ N reaction. This eases the problem of usingintense radiation sources in an airport environment. The strength of thesource used should be about 1% of those presently used.

An important feature of the device 1 is that it measures a key quantity,W_(R) or W (described below) which is independent of the thickness ofthe material within the container that is scanned.

Am--Be or 252-Cf or a pulsed accelerator are suitable for use as sourcesof radiation but other radioactive materials known to one skilled in theart can be used. The pulsed accelerator involves an energetic deuteronbeam (e.g. 2 MeV) incident for example on a deuterium gas target, atritium gas target, a Beryllium or Lithium target producing reactionsrespectively ² H(d,n)³ He; ³ H(d,n)⁴ He; ⁹ Be(d,n)¹⁰ B; ⁷ Li(d,n)⁸ Be,which are all neutron producing reactions.

The pulsed beam allows the separation of neutrons and gamma ray eventsin time since neutrons take longer times to travel to the detectors thanthe γ-rays. The beam can pulse as illustrated in FIG. 5 wherein theseparation of pulses is about 250 ns.

During the short burst periods fast neurons and γ-rays are detected.During the 100 μs period the γ-rays from neutron thermalisation andcapture by ¹⁴ N are detected. Detection during the 100 μs periodminimises interference from the direct deuteron beam which is off inthis period.

Detection of Drugs and Explosives

Several variations are possible:

Equations:

Neutron transmission through container;

    I.sub.n /I.sub.no =exp-{μ.sub.n1 m.sub.1 +μ.sub.n2 m.sub.2 }(1)

Gamma transmission through container:

    Iγ/Iγ.sub.o =exp-{μγ.sub.1 m.sub.1 +μγ.sub.2 m.sub.2 }                                                 (2)

Thickness of contents if pure explosive; ##EQU1##

density of contents of container if pure explosive; ##EQU2##

Contents "factor" ##EQU3## the data collected can be graphicallyrepresented as shown in FIGS. 6a and 6b.

Objects are scanned and moment by moment measurements made. Eachmeasurement involves the following:

The results of the analysis of the transmissions of the neutrons and thegamma rays as counted in the detector are represented in a two (or more)dimensional region called a W-RHO plane which is defined by thesoftware. This plane has certain mathematical characteristics which willnow be described.

A mixture of two standard materials (which may be fictitious) arecharacterized by certain mass attenuation coefficients. The real timemeasurements of this mixture can be represented as follows:

The composition is calculated from equation 5 (usually expressed as aweight percent (wt %), W) by multiplying the result from equation 5 by100. These wt % values define the Y values in the W-RHO plane. The Xvalues are calculated from the quantity given by equation 4 wherein x isthe effective thickness for the particular measurement. The effectivethickness is determined from the attenuation of the gamma ray componentalone. The expression:

    1n(I.sub.γo /I.sub.γ)=μ.sub.γ1 M1+μ.sub.γ2 M2 (6)

is used to estimate the amount of material in the beam. A value of thewt % is a "working value" that is adopted solely for the purposes of thethickness calculation. The masses per unit area of the two componentsare indicated here by M1 and M2 (large letters) to distinguish them fromthe NEUGAT estimates m₁ and m₂ deduced from equations 1 and 2.

    Now, wt % W=(M1/(M1+M2))*100                               (7)

and equations 6 and 7 can be used to solve for M1 and M2. The effectivethickness, x, can be calculated from:

    x=(M1/rho1)+(M2/rho2)                                      (8)

wherein rho1 is the density of pure material 1 and rho2 is the densityof pure material 2. It is this value "x" which is substituted intoequation 4 and defines the X axis of the W-RHO plane.

An operational line can be defined on the W-RHO plane but which is onlyrelevant when mixtures of the "standard" materials 1 and 2 are beinganalyzed. This operational line is called the "NEUGAT line" and isanalogous to the load line used to represent the range of values whichcan be taken up by electronic devices such as amplifiers. In general theline will be a curve defined by the equation: ##EQU4## The end pointsfor NEUGAT measurements are:

    when RHO=rho1 then W=100                                   (10)

    when RHO=rho2 then W=0                                     (11)

The NEUGAT line is shown in FIG. 6a. The region where data for a sampleconsisting of 30 wt % material 1 (ml) is also indicated in that Figure.If mixtures of the "standard" materials 1 and 2 are being analysed thedata will fall in a group for a certain mixture somewhere along theNEUGAT line.

The thicknesses of material in the beam can vary considerably so realtime corrections are preferably made to the mass attenuationcoefficients for each measurement in a sequence based on the effectivethickness for that measurement. For example:

(i) The effective thickness of the material is estimated from equation8.

(ii) The effective thickness value is used to correct the massattenuation coefficients based on a predetermined algorithm (usually thecoefficients reduce quadratically with increasing thickness).

(iii) The values of m1 and m2 are estimated using NEUGAT (equations 1and 2).

(iv) The composition W is determined using equation 5 and expressed as aweight percent.

(v) The results of (i) and (iii) are used to determine RHO.

(vi) The precision of the measurements of W and RHO can be calculatedusing a standard statistical analysis such as that described by Tominagaet al in International Journal of Applied Radiation and Isotopes, 34,429 (1983).

The procedure for making real time measurements of mixtures of the"standard" materials 1 and 2 using NEUGAT alone has been described thusfar, however, the importance of the W-RHO plane becomes apparent whendifferent mixtures are used.

The procedure that is now to be described enables real time measurementsof mixtures of any chemical combination to be made and interpreted.Every chemical combination will have a grouped response somewhere on thedefined W-RHO plane but of course that is now not confined to the NEUGATline since that was defined solely with respect to the "standard"materials 1 and 2.

The NEUGAT response is unique to each chemical so it can be thought ofin terms of a signature or fingerprint. Similar chemicals give similarresponses and in fact the distance between the (x,y) values for eachchemical grouping is a measure of how similar certain chemicals are.Explosives and drugs have characteristic features and thus distinctivesignatures that enable their detection by the invention method anddevice of the present invention. For example, explosives havecharacteristically high densities of nitrogen and hydrogen, and whiledrugs have lesser densities they usually have high hydrogenconcentrations. Experimentation will reveal the regions on the W-RHOplane that characterise other groups of chemically similar compoundssuch as prohibited materials.

Some regions of the W-RHO plane may overlap and so to enable accuratedeterminations the W-RHO plane can be extended to three or moredimensions by introducing other constraints such as neutron activation.

FIG. 6b shows a possible separation of explosives, drugs and clothesusing the W-RHO plane NEUGAT approach and FIG. 7 shows an extension tothree dimensions by including neutron activation data. Four or moredimensions can be used if necessary if a number of complementary methodsare are brought to bear on the problem.

The presence of nitrogen can be monitored in addition to neutron/gammatransmission using the ¹⁴ N(n,γ)¹⁵ N reaction. This is graphicallyillustrated in FIG. 7. C_(R) represents the nitrogen gamma count rateand:

I_(n) =transmitted neutrons;

I.sub.γ =transmitted gamma rays;

I_(no) =incident neutrons;

I.sub.γo =incident neutrons;

μ_(n1) =mass attenuation coefficient for reference material 1(neutrons);

μ_(n2) =mass attenuation coefficient for reference material 2(neutrons);

m₁ =areal density of reference material 1;

m₂ =areal density of reference material 2;

μ.sub.γ1 =mass attenuation coefficient for reference material 1 (gammarays);

μ.sub.γ2 =mass attenuation coefficient for reference material 2 (gammarays);

RHO=density;

rho1=density of reference material 1;

rho2=density of reference material 2;

μ.sub.γexp =mass attenuation coefficient for pure explosive (gammarays);

o_(exp) =density of explosive; and

The reference materials are chosen so that materials such as explosives,drugs are conveniently portrayed. The reference materials, which may behypothetical, are specified by the choice of the mass attenuationcoefficients μ_(1n), μ_(2n), μ₁γ and μ₂γ.

The positions of the "scan event points" depend on the "relativeamounts" of the reference materials present. In actuality, othermaterials may be present with different mass attenuation coefficients,but particular materials will be grouped in definite regions of the scanevent graphs.

The foregoing describes the invention including a preferred method anddevice thereof. Alterations and modifications as will be obvious to oneskilled in the art are intended to be incorporated within the scope ofthe invention which is defined by the following claims.

I claim:
 1. A method for detecting the presence of a number ofsubstances in a container, which comprises irradiating the containerwith fast neutron and gamma source radiation, measuring the extent towhich each species of the source radiation is transmitted through thecontainer, and analyzing the measurements with reference to knowncharacteristic attenuation coefficients and density properties of thesubstances for each species of the source radiation to determine fromthe attenuation of both the transmitted neutron and gamma sourceradiations the presence of said substances.
 2. A method according toclaim 1 wherein the container is irradiated by the fast neutron andgamma radiation substantially simultaneously.
 3. A method according toclaim 2 wherein the neutron and gamma source radiation have differentfrequencies so they can be readily distinguished.
 4. A method accordingto claim 3 wherein the source radiation is produced by a radioactiveisotopic source.
 5. A method according to claim 4 wherein theradioactive source is californium-252 or americium-beryllium.
 6. Amethod according to claim 3 wherein a number of containers are passedbetween irradiating means and transmission measurement means.
 7. Amethod according to claim 3 wherein thermal neutron activation and/orx-ray measurements are made in addition to the neutron and gammaradiation transmission measurements to aid the analysis.
 8. A methodaccording to claim 3 wherein the transmission measurements are analysedby data processing means and the results are displayed.
 9. A methodaccording to claim 8 wherein an alarm is triggered when at least one ofthe substances is detected.
 10. A method according to claim 3 whereinthe substances are drugs and/or explosives.
 11. A method according toclaim 10 wherein the container is a suitcase, bag or package. 12.Apparatus for detecting the presence of a number of substances in acontainer, which comprises means which irradiates the container withfast neutron and gamma source radiation, means which measures the extentto which each species of said radiation is transmitted through thecontainer, and means which analyses the measurements with reference toknown characteristic attenuation coefficients and density properties ofthe substances for each species of the source radiation and determinesfrom the attenuation of both the transmitted neutron and gamma sourceradiations the presence of said substances.
 13. Apparatus according toclaim 12 wherein the irradiating means emits the neutron and gammasource radiation substantially simultaneously.
 14. Apparatus accordingto claim 13 wherein the irradiating means emits neutron and gammaradiation that have different flight times or detector responses so theycan be readily distinguished.
 15. Apparatus according to claim 14wherein the irradiating means comprises a radioactive isotopic source.16. Apparatus according to claim 15 wherein the isotopic source iscalifornium-252 or americium-beryllium.
 17. Apparatus according to claim15 wherein the irradiating means and the transmission measuring meansare housed in a detecting unit.
 18. Apparatus according to claim 14which includes means which conveys a number of containers between theirradiating means and the transmission measuring means.
 19. Apparatusaccording to claim 18 wherein said analyzing means is a data processor.20. Apparatus according to claim 19 wherein said data processor has amonitor to display the results of the analysis.
 21. Apparatus accordingto claim 14 further including means which measures thermal neutronactivation of the substances in the container and/or x-ray measurementmeans to aid the analysis.
 22. Apparatus according to claim 14 furtherincluding an alarm system that is triggered when at least one of thesubstances is detected.
 23. Apparatus according to claim 14 which candetect the presence of drugs and/or explosives in suitcases, bags andpackages.